U.S. Pat. No. 4,263,128 discloses a selective vaporization process which reduces the Conradson Carbon and metals content of heavy hydrocarbon feedstocks destined for catalytic cracking or other hydrocarbon conversion processes. Broadly, the selective vaporization system comprises a contactor, a burner/regenerator and an inventory of circulating, fluidizable, essentially inert contact material. Rapid application of heat, generated within the burner and carried by the circulating inventory of inert contact material, selectively vaporizes most of the hydrocarbon components of the feed. During this selective vaporization, non-vaporized components of the feedstock, including metals, are deposited on the contact material. The spent contact material, bearing deposits of Conradson Carbon and metal components of the hydrocarbon feedstock, is then introduced into a burner/regenerator where combustible portions of the deposited materials are oxidized. The contact material is then recycled to the contactor. Because contaminating metal deposits are not concurrently removed in the regenerator, the quantity of metal contaminants on a particle of contact material typically increases during each of its cycles through the selective vaporization unit.
The accumulated metals that are deposited on the contact material during selective vaporization can have undesirable effects upon the properties of the material. For example, excessive quantities of vanadium on the contact material can cause the material to agglomerate in the selective vaporization unit. In practice, in selective vaporization processes, the metals content of the selective vaporization contact material has been held at acceptable levels by withdrawing a portion of that material on a continuous or semi-continuous basis and replacing it with fresh contact material. In order to maintain the overall efficiency of selective vaporization processes, it would be desirable to remove the metals from the contact material that is withdrawn from the selective vaporization unit and then to recycle the metals-depleted contact material back to the selective vaporization unit. In addition, it would be desirable to recover the metals removed from the contact material in saleable form.
The '128 patent discloses that the preferred contact material for selective vaporization comprises calcined kaolin clay, an aluminosilicate material. Metals which deposit and build up on the surface of such contact material during the selective vaporization step tend to combine chemically with the material during the combustion step. Separation of those metals from the contact material is difficult, often leading to degradation of the material itself during the metals-removal process.
Metals extraction processes which lead to co-extraction of alumina or silica are not useful in treating aluminosilicate materials, such as calcined kaolin clay, because such processes adversely affect the integrity of the materials. For example, approximately 90% vanadium recovery could be obtained by boiling the particles in 10% caustic soda but the particles are degraded and become essentially useless as a contact material thereafter. Decreasing the severity by lowering the temperature or concentration of caustic decreases the vanadium recovery significantly but the amounts of alumina and silica extracted are sufficient to interfere with subsequent liquid ion extraction processes for recovering the metal. As another example, strong mineral acids will extract nickel and vanadium from aluminosilicate contact materials, such as microspheres of calcined kaolin clay, but alumina is co-extracted, leading to a reduction in the amount of material available for recycling, an undesirable increase in the surface area of the microspheres and decreased attrition resistance depending upon the amount of alumina extracted.